Charge Transfer Pathways in Three Isomers of Naphthalene-Bridged Organic Mixed Valence Compounds

Naphthalene was substituted at different positions with two identical triarylamine moieties to result in species which are mixed-valence compounds in their one-electron oxidized forms. They were investigated by cyclic voltammetry, optical absorption, EPR spectroscopy, X-ray crystallography, and DFT calculations. When the two redox-active triarylamine moieties are connected to the 2- and 6-positions of the naphthalene bridge, their electronic communication is significantly stronger than when they are linked to the 1- and 5-positions, and this can be understood on the basis of a simple through-bond charge transfer pathway model. However, this model fails to explain why electronic communication between triarylamine moieties in the 1,5- and 1,8-isomers is similarly strong, indicating that through-space charge transfer pathways play an important role in the latter. In particular, charge transfer in the 1,8-isomer is likely to occur between the triarylamino C atoms in α-position to the naphthalene linker because the respective atoms are only about 3 Å apart from each other, and because they carry significant spin density in the one-electron oxidized forms of triarylamines. This particular through-space charge transfer pathway might be generally important in molecular structures based on the 1,8-disubstituted naphthalene pillaring motif.